Why do you need a CV Cover Letter?
A good covering letter will literally cover any problem areas and complex issues and explain them - thus making the CV more likely to be read.
Covering letters introduce your CV. They are there to induce the reader to go on and read the CV, and to explain away any 'difficult' areas in the CV that might otherwise be off-putting.
They tell the reader why you want the job, why they should interview you, and that you are available. A good covering letter will make the impression that your CV is a good one, and that you are therefore an excellent candidate. It allows you the room to explain anything that isn't obvious, that needs clarification, etc.
If the CV is a sales pitch for yourself, the covering letter is a sales pitch for the CV. The CV is a tool to get you an interview, and the covering letter a tool to get the CV noticed for the right reasons. Preparing a good, personalised covering letter takes time, but it is a worthwhile investment.
How do you make an impression? Remember, there may be hundreds of applications arriving with yours, so how do you make yours one of the ones selected?
Show interest. It's surprising how many covering letters don't actually say they want the job. Show you're interested.
e.g. I was extremely interested to see your advertisement for the above position.
or I think this would be an exciting opportunity.
Clear up problem areas. Clear up anything in the CV that is ambiguous, or needs explaining, clearly and concisely. Don't be apologetic.
Put yourself forward. Don't be shy. If you can, pick out some skills or experience they are looking for that you have, and mention it here. Give them a reason to choose you over other candidates.
e.g. I have three years' experience in a similar role, and am now ready for the greater responsibility offered by this post.
Give a reason for applying.
e.g. I have been interested in technology for some time, and would welcome an opportunity to move into a more progressive environment.
Flatter them - but carefully. There's nothing wrong with a little flattery - so long as you don't overdo it. Mention things you like about their company if relevant. But DON'T go on and on about it. Too much is worse than nothing.
e.g. I have always been interested in your innovative marketing, and would like to join a team working with such success in developing new ideas.
Don't grovel. It's surprising how many candidates adopt a 'please, please consider me, if it's not too much trouble' attitude. It makes you look as if you are desperate for the job, and lacking in confidence. Take the position that you want the job, you're right for it, and they should be looking at you. Be confident enough that it shows.
Don't repeat the CV. Give new information. The covering letter is an opportunity to show 'soft skills' that may not come across in your CV, such as interpersonal skills, teamwork, maturity, etc.
Keep it short. One side of A4 is quite sufficient. You don't need 2 pages, and many recruiters won't reach that much anyway.